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What's the best motor on variable speed drives?

The key to know which motor is better to use on variable speed drives depends entirely on the individual application for which it will be used.

You need to ask questions such as: 1. what will the cable distance be between the drive and the motor? 2. What is the system voltage? 3. What is the rise time of the variable speed drive’s output IGBTs? 4. Is the load constant torque, variable torque or constant horsepower? 5. What speed range is required? 5. Will the motor always be powered by the VSD or is across-the-line bypass required? 6. What is the motor's nameplate voltage and frequency rating? 7. What is the horsepower or kW rating of the motor? 8. Will the conductors between the motor and the AC drive be shielded or non-shielded? 9. What type of raceway will be used for the conductors between the VFD drive and the motor?

The key to picking the proper motor based on the answers to these questions is to understand the difference between NEMA MG1 Part 30 which is titled "Application Considerations for Constant Speed Motors Used on a Sinusoidal Bus with Harmonic Content and General Purpose Motors Used with Adjustable-Voltage or Adjustable-Frequency Controls or Both" and NEMA MG1 Part 31 which is titled "Definite-Purpose Inverter-Fed Polyphase Motors".

For systems that are rated for 230 volts generally do not have a problem with reflected waves because the motor insulation systems are generally rated for 600V systems anyway. Reflected waves on 230V systems would have peak voltages no greater than 650 to 1000 volts.

On 380V or higher systems, if the cable distance between the variable speed drive and the motor is relatively short (typically 25 feet or less) then there need not be any concern about reflected waves damaging the motor insulation. On retrofit situations where you need to re-use an existing motor and you can't determine the peak voltage withstand rating of the motor's insulation system the you can add devices either at the VFD drive output or at the motor which can prevent reflected wave voltage spike damage.

NEMA MG1 Part designates that for motors with a nominal insulation rating of equal to or greater than 600V that the peak voltage rating of the insulation system has to be equal to or greater than 1kV with a rise time of greater than or equal to 2 microseconds.

NEMA MG1 Part designates that for the same nominal voltage rating as above the peak voltage rating of the insulation system needs to be 3.1 times Vrated with a rise time of equal to or greater than 0.1 microseconds. There is some debate as to whether Vrated in a nominal 480V system is 480V or the motor nameplate line-to-line voltage rating of 460V. The difference is relatively small, 1426V vs. 1488V.

The operating speed range has more to do with determining the ability of the motor to keep itself operating below its rated temperature rise when operated below its nameplate speed than with anything else. This is more of a problem for constant torque applications with very wide speed ranges that for any other type of application. This is especially true if the application requires that the motor operate at rated torque at zero speed as in some web handling applications.

There are some "vector-duty" motors that have very wide C.T. speed range capabilities but are not suitable for applications that require across-the-line bypass operation. That may be because they are really a NEMA Design A motor which has similar locked rotor torque capabilities to a NEMA Design B motor but the locked-rotor current is not limited by NEMA as it is in a NEMA Design B motor. This could cause upstream short-circuit devices to trip or fuses to blow when the motor is started in the bypass mode. Another consideration is that there are some "vector-duty" motors that are designed with non-standard volts-per-hertz ratios such as 460V/90Hz.

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